In the PAL system, the hue error caused by transmission distortion can be cancelled. An (R-Y) signal, one of the two kinds of color difference signals, is transmitted so that the phase of the (R-Y) is inverted every scanning line. A demodulator is used to cancel hue error of the chrominance signal by demodulating the chrominance signal using an operation between scanning lines at the receiver side. In this color demodulator, a 1H delay device is used for the demodulation operation between scanning lines. Here, 1H is one horizontal period. In another type of color demodulator, no delay device is used and color signals are demodulated at each scanning line as in the NTSC system. Hue error between adjacent scanning lines is cancelled by an integral effect of the viewer's eyes.
A block diagram of a prior art color demodulation apparatus for the PAL system using a delay device is shown in FIG. 1.
An input chrominance signal is delayed by 1H at a 1H delay device 203. The input chrominance signal and the 1H delayed chrominance signal are added at a first adder 205. In the PAL system, because the phase of the (R-Y) signal component is inverted every scanning line, the modulated (R-Y) signal components on adjacent scanning lines are cancelled with each other at the first adder 205 and only a (B-Y) signal component is outputted. The chrominance signal is phase inverted at a phase inverting circuit 204 and is added to the 1H delayed chrominance signal at a second adder 206. At the second adder 206, the (B-Y) signal components on the adjacent scanning lines are cancelled with each other and only an (R-Y) signal component is outputted.
A subcarrier generator 202 generates a subcarrier signal (4.43MHz) synchronized with a burst signal included in the input composite color television signal. The subcarrier signal is phase shifted by 90 degrees at a phase shift circuit 207 and is inputted to a (B-Y) demodulator 209 together with a modulated (B-Y) signal from the first adder 205 to produce the (B-Y) signal.
The subcarrier signal is inverted every scanning line at a line switching circuit 208. In the PAL system, the burst signal alternates its phase by +135 degrees and -135 degrees against the reference axis (B-Y) every scanning line. A line alternating signal is generated, based on the compared result between the phases of a burst signal and the subcarrier signal. At the line switching circuit 208, the subcarrier signal is phase shifted by 180 degrees every scanning line, synchronous with the line alternating signal.
The output of the line switching circuit 208 is inputted to an (R-Y) demodulator 210 together with the modulated (R-Y) signal which is output from the second adder 206 to produce an (R-Y) signal. This color demodulation method can cancel the phase distortion of the chrominance signal which occurs during transmission using an operation between scanning lines.
This color demodulation method is called PAL-D.
A block diagram of another prior art type color demodulation apparatus for the PAL system, in which a delay device is not used, is shown in FIG. 2. A subcarrier generator 302, a phase shift circuit 303, a line switching circuit 304, a (B-Y) demodulator 305, and an (R-Y) modulator 306 operate similarly to the blocks having the same names, and identified by reference numerals 202, 207, 208, 209, and 210 in FIG. 1, respectively.
This color demodulation method does not use a 1H delay device and can cancel a hue error between two signals on adjacent scanning lines utilizing an integral effect of the viewer's eyes. This color demodulation method is called PAL-S.
A color demodulation apparatus using a delay device has a problem. If there is no correlation between the signals on adjacent scanning lines, correct color demodulation does not occur because correlation of the phase distortion is performed between adjacent scanning lines. The color demodulation apparatus not using the delay device has a problem in that when phase error increase, venetian blind interference appears on the screen.